A More Natural Approach to Passive Ankle Prostheses

A research lab has found an innovative way to close the gap between low-tech passive ankle prostheses and high-tech robotics.

by Cassie Kelly

January 29, 2018

A morning walk around the park is often an effortless and peaceful experience for an able-bodied person. Going up small slopes, speeding up and slowing down pace, or even just stopping to stare at a tree may all seem like simple motions. But, the human ankle and foot that control these actions are a highly intricate and powerful duo and mimicking them in the form of a prosthetic is an incredible engineering challenge.

At SpringActive Inc., a collaborative research lab in Tempe, Arizona known for their robotic leg prosthetic SPARKY, researchers believe they have found an innovative way to revolutionize passive ankle prostheses to make those walks in the park easier for amputees without all of the necessary motors and batteries of a robotic prosthetic.

“There’s a huge gap between the capabilities of robotic and passive prostheses,” Robert Holgate, former design engineer at SpringActive, said. “We’re just trying to close that gap while keeping the design affordable and accessible.”

The Lightweight Energy Assistive Foot (LEAF) is a joint project between SpringActive and Arizona State University. The LEAF’s design incorporates a revolving spring ankle joint that matches the ground slope during heel strike in a gait cycle and stores the energy on impact. Then when the user rolls over the ankle, the breaking energy that was stored is released to assist in the push off of the step. When the prosthetic is in swing phase, it then resets the entire system for the next heel strike.

“It must be totally comfortable for the amputee,” Holgate, the lead engineer of the design, said. “So, at heel strike where the user lands on the foot, we don’t want them to feel like they’re stepping in a hole. We also don’t want them to land too early and feel like they're hitting a bump, which might throw them off balance.”

To smooth out this process, Holgate and the team incorporated two components into the ankle joint. The lower system uses a small soft spring to absorb the impact of the heel strike while integrating to the angle of the ground. That compression of the heel strike is then released into the toe until it falls to the ground. Then, a set of gears will lock the foot in place and one-way bearings will turn the other direction, transitioning the foot to the upper mechanism. At this stage in the gait cycle, the person will begin to roll over their ankle at which point a larger spring in the upper system, which is tuned to the person’s weight, will absorb and store the energy into potential energy. When the person reaches the point where the spring will take over, that stored energy will propel the ankle forward. It’s important to note that if the lower system was not locked the whole time, all of the energy absorbed at heel strike would be lost and the toe would move freely, never transitioning to the upper system. So, both systems must work in harmony to make walking as natural as possible.

The LEAF is very light, hence the name. It is equipped with a carbon fiber frame and every gram is accounted for in the design. It also will need very little customization for each user. The upper system spring can be changed out for a smaller or larger one and the tension of the spring can be fitted to a user’s preference.

“It’s like when we buy shoes, some people like it to be more supportive and others like it to be more nimble and feel softer,” said Holgate. “Some people walk differently and use their muscles differently too. Truly, it just has to match the strength of their existing ankle.”

Another perk of the LEAF is that it will have a relatively long lifespan of about five years, which in prosthetic terms, is quite some time.

“If you think about the foot like a car, a car can typically run up to 100 miles before it really starts failing,” Thomas Sugar, co-founder of SpringActive and engineering professor at ASU, said. “But that’s nothing compared to a foot that takes five to ten thousand steps a day for 365 days for five years straight. It’s hard to believe how amazing the human body really is when you put it in these terms.”

The LEAF is currently being funded by a 6-month Phase I STTR grant to create a viable prototype. They’re main focus right now is perfecting the the locking mechanism to make sure that it aligns and locks properly no matter where the ankle bearing is in the gait cycle. At the end of the 6-month period, the team will aim for a Phase II grant for human trials and gearing up for commercialization.

Sugar said the prosthetic should be affordable and potentially even covered by most insurance providers because of its lifespan and little need for customization or repair. With some luck, the team will be able to continue their efforts on the LEAF and it will be a unique option for amputees.

“The goal is to help them be as asymmetric as possible to reduce injuries to their back and strain on their muscles,” Holgate said. “We want the user to be back to normal functionality so they won’t even know they're missing an ankle.”

Cassie Kelly is a science and technology writer based in New York City.